I did provide the information for it, Dropbox and Microsoft talks at CppCon, easy with some Google fu.
CppCon 2014: Zaika Antoun "Microsoft w/ C++ to Deliver Office Across Different Platforms, Part I"
https://www.youtube.com/watch?v=3HROqnw-nf4
CppCon 2014: Zaika Antoun "Microsoft w/ C++ to Deliver Office Across Different Platforms, Part II"
https://www.youtube.com/watch?v=MGMoRu5yrVc
CppCon 2015: Jacob Potter & Andrew Twyman “Bridging Languages Cross-Platform..."
https://www.youtube.com/watch?v=K-k-axW2utc
CppCon 2017: Stephen Spann “Cross-Platform Apps with Dropbox’s Djinni...”
https://www.youtube.com/watch?v=ssqhz_1pPI4
CppCon 2014: T. Grue & S. Kabbes "A Deep Dive into 2 Cross-Platform Mobile Apps Written in C++"
https://www.youtube.com/watch?v=5AZMEm3rZ2Y
Naturally the native UI has to be used.
Are we talking about having a good user experience here, or having the easy lazy path for the developer?